When a multi-tone signal is amplified, as is the case in a wide variety of applications, undesirable intermodulation distortion (IMD) products are inherently produced, resulting in distortion of the amplified output signal. These IMD products are particularly troublesome in wireless communications applications where signals of several frequency channels are often amplified by a common amplifier. Without proper reduction of the IMD products, signal interference between adjacent channels or within a single channel can become excessively high.
In order to achieve low intermodulation distortion, the amplifier typically has to be backed off from its thermally rated average power output and linearized. In wireless applications, both of these approaches add significantly to the high cost of the transmitting amplifier.
RF amplifiers have been linearized in the past via the use of either predistortion or feed-forward methods of canceling intermodulation. Feed forward amplifier circuits typically employ a main amplifier which produces fundamental and unwanted IMD power, along with a correction amplifier to produce only IMD power. The IMD power of the two amplifiers are then cancelled in an output combiner. While this feed-forward technique is satisfactory for some systems, it is very expensive and requires critical alignment. An example of a feed-forward, low distortion amplifier can be found in U.S. Pat. No. 5,304,945 entitled "Low-Distortion Feed-Forward Amplifier", which is assigned to the assignee herein.
Predistortion methods to reduce IMD have also been utilized in the prior art. In a predistortion amplifier circuit, the input signal is split into two paths: a direct path and a predistorter path. In the predistorter path, the input signal is conditioned in some manner to produce a predistorter signal that contains some signal energy at IMD frequencies. This predistortion signal is then combined with the signal in the direct path, and the combined signal applied to a main amplifier. The output signal of the main amplifier then will have less distortion than it would without the predistorter signal, provided that the amplitude and phase of the predistorter signal is properly selected.
An example of a prior art predistortion amplifier can be found in U.S. Pat. No. 4,157,508 entitled "Signal Cuber Circuit". The amplifier circuit in this patent utilizes a pair of anti-parallel diodes in the predistorter path to generate signal energy at the IMD frequencies. This diode arrangement creates a "cuber"--i.e., a circuit which produces signals at the third order IMD frequencies. The cuber disclosed therein used a balance bridge to minimize the signal leak-through at the fundamental frequencies, and an additional resistor to minimize the 5th order distortion in the cuber output. However, for signal-to-noise (SNR) considerations, diodes with large reverse saturation current had to be used, with associated large junction capacitance. This choice limited the frequency response of the diodes, thus preventing that cuber's use at high frequencies such as in standard cellular telephone bands.